The Roles of CDPKs as a Convergence Point of Different Signaling Pathways in Maize Adaptation to Abiotic Stress.

The calcium ion (Ca2+), as a well-known second messenger, plays an important role in multiple processes of growth, development, and stress adaptation in plants. As central Ca2+ sensor proteins and a multifunctional kinase family, calcium-dependent protein kinases (CDPKs) are widely present in plants. In maize, the signal transduction processes involved in ZmCDPKs' responses to abiotic stresses have also been well elucidated. In addition to Ca2+ signaling, maize ZmCDPKs are also regulated by a variety of abiotic stresses, and they transmit signals to downstream target molecules, such as transport proteins, transcription factors, molecular chaperones, and other protein kinases, through protein interaction or phosphorylation, etc., thus changing their activity, triggering a series of cascade reactions, and being involved in hormone and reactive oxygen signaling regulation. As such, ZmCDPKs play an indispensable role in regulating maize growth, development, and stress responses. In this review, we summarize the roles of ZmCDPKs as a convergence point of different signaling pathways in regulating maize response to abiotic stress, which will promote an understanding of the molecular mechanisms of ZmCDPKs in maize tolerance to abiotic stress and open new opportunities for agricultural applications.

A new adenylyl cyclase, putative disease-resistance RPP13-like protein 3, participates in abscisic acid-mediated resistance to heat stress in maize.

In plants, 3´,5´-cyclic adenosine monophosphate (cAMP) is an important second messenger with varied functions; however, only a few adenylyl cyclases (ACs) that synthesize cAMP have been identified. Moreover, the biological roles of ACs/cAMP in response to stress remain largely unclear. In this study, we used quantitative proteomics techniques to identify a maize heat-induced putative disease-resistance RPP13-like protein 3 (ZmRPP13-LK3), which has three conserved catalytic AC centres. The AC activity of ZmRPP13-LK3 was confirmed by in vitro enzyme activity analysis, in vivo RNAi experiments, and functional complementation in the E. coli cyaA mutant. ZmRPP13-LK3 is located in the mitochondria. The results of in vitro and in vivo experiments indicated that ZmRPP13-LK3 interacts with ZmABC2, a possible cAMP exporter. Under heat stress, the concentrations of ZmRPP13-LK3 and cAMP in the ABA-deficient mutant vp5 were significantly less than those in the wild-type, and treatment with ABA and an ABA inhibitor affected ZmRPP13-LK3 expression in the wild-type. Application of 8-Br-cAMP, a cAMP analogue, increased heat-induced expression of heat-shock proteins in wild-type plants and alleviated heat-activated oxidative stress. Taken together, our results indicate that ZmRPP13-LK3, a new AC, can catalyse ATP for the production of cAMP and may be involved in ABA-regulated heat resistance.

The calcium-dependent protein kinase ZmCDPK7 functions in heat-stress tolerance in maize.

Global warming poses a serious threat to crops. Calcium-dependent protein kinases (CDPKs)/CPKs play vital roles in plant stress responses, but their exact roles in plant thermotolerance remains elusive. Here, we explored the roles of heat-induced ZmCDPK7 in thermotolerance in maize. ZmCDPK7-overexpressing maize plants displayed higher thermotolerance, photosynthetic rates, and antioxidant enzyme activity but lower H2 O2 and malondialdehyde (MDA) contents than wild-type plants under heat stress. ZmCDPK7-knockdown plants displayed the opposite patterns. ZmCDPK7 is attached to the plasma membrane but can translocate to the cytosol under heat stress. ZmCDPK7 interacts with the small heat shock protein sHSP17.4, phosphorylates sHSP17.4 at Ser-44 and the respiratory burst oxidase homolog RBOHB at Ser-99, and upregulates their expression. Site-directed mutagenesis of sHSP17.4 to generate a Ser-44-Ala substitution reduced ZmCDPK7's enhancement of catalase activity but enhanced ZmCDPK7's suppression of MDA accumulation in heat-stressed maize protoplasts. sHSP17.4, ZmCDPK7, and RBOHB were less strongly upregulated in response to heat stress in the abscisic acid-deficient mutant vp5 versus the wild type. Pretreatment with an RBOH inhibitor suppressed sHSP17.4 and ZmCDPK7 expression. Therefore, abscisic acid-induced ZmCDPK7 functions both upstream and downstream of RBOH and participates in thermotolerance in maize by mediating the phosphorylation of sHSP17.4, which might be essential for its chaperone function.